The International Information Center for Structural Engineers

Friday, 22 February 2019 01:00cat

New lightweight material has the strength of titanium

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A microscopic sample of "Metallic wood" A microscopic sample of "Metallic wood"

Researchers have developed a sheet of nickel with nanoscale pores that has the strength of titanium but is 4-5 times lighter.

The new study was carried out by researchers at the University of Pennsylvania's School of Engineering and Applied Science, the University of Illinois at Urbana–Champaign, the University of Cambridge, and Middle East Technical University in Ankara. It was recently published in Nature Scientific Reports.

The high strength of metals derives from their atomic structure. However, flaws during the manufacturing procedure may significantly reduce their potential strength. Titanium is frequently utilized in structural industry as it presents similar strength with steel but it is twice as light. However, a titanium sample in which all atoms are perfectly aligned with its neighbors would be 10 times stronger than conventionally produced.

Researchers have achieved to work in the microscale in order to design a material that has an astonishing strength-to-weight ratio. They managed to design geometrical structures with reduced nanoscale defects.

Their approach includes suspending small plastic spheres (a few hundred nanometers in diameter) in water. When the water evaporates, the spheres settle creating a crystalline framework. Then, they infiltrate the plastic spheres with nickel and the spheres are dissolved with a solvent. Finally, an open network of metallic struts is left. Due to fact that 70% of the produced material is empty space, the material is extremely light in comparison to its strength.

The new material was named "Metallic Wood" due to its pores that are made in the same way as a natural material like wood. The empty space created by its porosity can be filled with other materials that can serve other purposes such as wood porosity is utilized for transporting energy. "The reason we call it metallic wood is not just its density, which is about that of wood, but its cellular nature. Cellular materials are porous; if you look at wood grain, that's what you're seeing – parts that are thick and dense and made to hold the structure, and parts that are porous and made to support biological functions, like transport to and from cells," James Pikul, Assistant Professor in the Department of Mechanical Engineering and Applied Mechanics at Penn Engineering and leader of the study, stated. "Our structure is similar. We have areas that are thick and dense with strong metal struts, and areas that are porous with air gaps. We're just operating at the length scales where the strength of struts approaches the theoretical maximum," Prof. Pikul added.

According to researchers, the process could be further optimized utilizing 3D-printing techniques that would enable higher accuracy but this procedure cannot be implemented in large scale and therefore it is not beneficial. "We've known that going smaller gets you stronger for some time but people haven't been able to make these structures with strong materials that are big enough that you'd be able to do something useful. Most examples made from strong materials have been about the size of a small flea, but with our approach, we can make metallic wood samples that are 400 times larger," Prof. Pikul commented.

The next phase of the research is to produce larger samples that can be subjected to conventional tests. A critical issue that must be investigated is the tensile properties of the material. "We don't know, for example, whether our metallic wood would dent like metal or shatter like glass. Just like the random defects in titanium limit its overall strength, we need to get a better understand of how the defects in the struts of metallic wood influence its overall properties," Prof. Pikul said.



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